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Abstract

Copulatory organs rapidly evolve and are subject to complex selective pressures affecting mating success. One feature of copulatory organs that is subject to such selective pressures is size. Benefits of longer organs may include greater signal effectiveness in courtship and longer ‘reach’ when attempting copulations with evasive females. Costs of longer organs may include impaired locomotion, increased energetic cost or reduced mechanical compatibility with female genitalia. The optimal size for a copulatory organ may vary with mating behavior. The objective of this study is to examine among-species variability in copulatory organ size, body size and the relationship between copulatory organ size and body size, in a closely-related set of six species within the livebearing fishes (Poeciliidae). The copulatory organ in the Poeciliidae is called the gonopodium. Among these fishes there are pronounced differences in mating behavior. In some species courtship often precedes copulation, especially when the male is relatively large. In other species there is no courtship and males attempt to force copulate by inseminating evasive females. I hypothesize that species that court will have either a shorter or longer gonopodium than species that do not court. Because small males of species that court and force copulate have less “need” for a long gonopodium, I hypothesize that gonopodia in large males in species with courtship will be disproportionately short. Variation in functionality of the anal fin between the sexes also suggests variation in the scaling relationship between body size and anal fin size. The findings revealed hypoallometry in all male species examined as well as a longer absolute and relative gonopodium size in species that perform both courtship and forced copulations. Female anal fins of all species were found to scale isometrically to body size.